1. Field of the Invention
The present invention relates generally to a remote plasma source for exciting a process gas into a plasma state. More particularly, the present invention relates to a remote plasma source for delivering excited gas species into a processing chamber in which a substrate is to be processed.
2. Background of the Related Art
Plasma assisted chemical reactions have been widely used in the semiconductor and flat panel display industries. One example is plasma-enhanced chemical vapor deposition (PECVD), which is a process that is used in the manufacture of integrated circuits and thin film transistors (TFT) for active-matrix liquid crystal displays (AMLCDs). hi accordance with capacitively coupled PECVD, a substrate is placed in a vacuum deposition chamber that is equipped with a pair of parallel plate electrodes. One of the electrodes, e.g., the lower electrode, generally referred to as a susceptor, holds the substrate. The other electrode, i.e., the upper electrode, functions as a gas inlet manifold or shower head to deliver gases into the chamber. During deposition, a reactant gas is supplied into the chamber through the upper electrode and a radio frequency (RF) voltage is applied between the electrodes to produce a plasma within the reactant gas. The plasma provides energy and enhances chemical reactions.
Although such systems are designed to preferentially deposit the material onto the surface of a substrate, they also deposit material onto other interior surfaces within the chamber. Consequently, after repeated use, the deposited layer of material that has built up in the chamber must be removed, typically using an in-situ dry cleaning process. According to the in-situ technique, precursor gases are supplied to the chamber. Then, by locally applying a glow discharge plasma to the precursor gases within the chamber, reactive species are generated. The reactive species clean the chamber surfaces by reacting with the deposits to form volatile compounds that can be removed in their gaseous state.
However, this in-situ cleaning technique has several disadvantages. First, it is inefficient to use a plasma within the chamber to generate the reactive species. Thus, it is necessary to use relatively high powers to achieve an acceptable cleaning rate. The high power levels, however, tend to produce damage to the hardware inside of the chamber, thereby significantly shortening its useful life. Since the replacement of the damaged hardware can be quite costly, this can significantly increase the per substrate cost of product that is processed using the deposition system. In the highly competitive semiconductor fabrication industry where substrate costs are critical, the increased operating costs resulting from periodic replacement of parts that are damaged during the cleaning process is undesirable.
Another problem with the conventional in-situ dry cleaning processes is that the high power levels that are required to achieve acceptable cleaning rates also tend to generate residues or byproducts that can damage other system components or which cannot be removed except by physically wiping off the internal surfaces of the chamber. For example, in a Si.sub.3 N.sub.4 deposition system which uses NF.sub.3 for cleaning, N.sub.x H.sub.y F.sub.z compounds tend to be generated. These ammonium compounds deposit in the vacuum pump where they can negatively affect the reliability of the pump used to create and maintain the vacuum environment in which the substrates are processed.
Deposition chambers or process kit components (e.g., a heater, shower head, clamp ring, etc.) made of ceramic or aluminum are often cleaned using an NF.sub.3 plasma, which contains excited F*.sub.(Gas) species. During this cleaning process, a certain amount of Al.sub.x F.sub.y is formed on the exposed surfaces of the chamber and the process kit components. The amount of Al.sub.x F.sub.y that is formed is greatly increased by the level of ion bombardment that results from the high plasma energy levels. Thus, a considerable amount of Al.sub.x F.sub.y can be formed in the system. Unfortunately, this material cannot be etched away by any known chemical process, so the chamber must be shut down and opened so that the deposits can be physically removed by wiping the interior surfaces of the chamber.
U.S. Pat. No. 4,988,644 discloses a remote plasma generator having a cooling jacket. However, the cooling jacket is limited to the gas tubes on the gas input tube 266 and quartz outlet tube 256 of the resonant cavity 260. The gas passage 270 which runs through the resonant cavity 260 and the resonant cavity 260 itself are not cooled. Consequently, the temperature of the quartz tube within the resonant cavity is still not controlled.
U.S. Pat. No. 5,262,610 discloses another remote plasma generator having a cooling system which utilizes a double wall quartz inlet tube 20 and a water cooled jacket 56 which couples the microwave applicator 54 to a matching device 52. However, the cooling jacket 56 does not extend around the portion of the gas tube 32 that is disposed within the microwave applicator 54. Therefore, the hottest region of the gas tube, which is the most likely region to crack and/or generate particles, extends into the microwave applicator where there is no cooling.
Therefore, there is a need for a remote plasma generator with a microwave resonant cavity and gas tube that can be cooled. It would be desirable if the cooling could be provided adjacent to the gas tube along the entire length of the resonant cavity. It would be further desirable if the generator provided tuning and matching adjustments to obtain efficient operation over a wide range of pressures and reactant gases.